1. The Field of the Invention
The present invention is related to ceramics which possess particularly high oxygen ion conductivity. More particularly, the present invention is related to a doped ceramics and methods and apparatus for their use in removing oxygen and water from a gaseous mixture of oxygen, water and other relatively inert gases.
2. Technical Background
In a number of applications it is important to remove oxygen from a mixture of gases. For example, even in purified gases, it is known that trace quantities of oxygen remain within the gas. In order to provide a very pure gas, it would be desirable to remove as much of the trace oxygen as possible. Examples of such gases include nitrogen and noble gases.
Small quantities of oxygen mixed within otherwise pure inert gases have proven problematic in a number of contexts. For example, in the manufacture of semiconductor devices, it is important to provide an essentially oxygen-free environment during certain types of processing steps. A typical solution for the problem is to flush the processing environment with an inert gas. However, even when inert gas fills the processing environment, trace amounts of oxygen still exist and are mixed with the inert gases.
Various processes have been attempted to remove oxygen from such inert gases. For example, it has been conventional to filter the gas in order to attempt to remove oxygen. Various filtering and removal processes have been employed, including adsorption, absorption, catalytic reactions, and membrane separation. Even using these processes, however, gases of less than ideal purity have been produced. Furthermore, such processes are cumbersome and difficult to use in large scale operations.
A reverse of the problem described above is involved in the production of commercial quantities of extremely pure oxygen. Problems similar to those described concerning other gases are also encountered in the production of pure oxygen. In all existing processes, it would be desirable to provide oxygen of better quality using a simple and relatively inexpensive process.
While not commonly used in gas purification, electrochemical devices which employ oxygen ion conducting electrolytes are known to exist. These electrolytes are widely used as oxygen sensors. Such sensors have received wide acceptance in devices such as automobile engines and furnaces where it is critical to maintain the ratio of fuel and oxygen within particular acceptable ranges. Some devices of this nature have also been employed for the purposes of preparing pure oxygen.
The mechanism of oxygen ion conduction is well known. Indeed, ionic conductivity of certain materials was studied by Nernst as early as the 1890's. Nernst found that if there was a difference in oxygen concentration across a dense zirconia membrane, an electrical potential could be measured from electrodes placed on opposite sides of the zirconia. Nernst showed that the following equation relates the applied voltage to the difference in oxygen concentration: EQU E=(RT/ZF) ln(p2/p1)
where: E=electrical potential (volts)
R=gas constant PA0 T=temperature PA0 Z=charge PA0 F=faraday constant PA0 p1=partial pressure oxygen on one side PA0 p2=partial pressure oxygen on the opposite side
Nernst also found that if a potential is applied across the membrane, oxygen ions can be transported from one side of the membrane to the other. The general mechanism of oxygen ion conductivity is believed to be as follows: EQU O.sub.2 +4e.sup.31 .fwdarw.2O.sup.-- 2O.sup.-- O.sub.2 .fwdarw.+4e.sup.-
It has been discovered, however, that conventional ceramics, such as zirconia, are inefficient at conducting oxygen ions. Pure zirconia, for example, is not generally incorporated into commercial gas purification devices. In addition zirconia is known to be difficult to handle. This is the case because pure zirconia ceramic experiences a phase transition from a monoclinic to a tetragonal structure at about 1170.degree. C. This results in a large change in volume, which in turn causes stress and cracking in dense ceramic parts.
In order to avoid some of the problems encountered with pure zirconia ceramics, it is conventional to add a dopant to the ceramic. Dopants are found to stabilize the tetragonal zirconia crystal structure. When zirconia is doped to higher levels, the structure can be stabilized in the a cubic phase. These materials are much easier to handle than pure zirconia. Yet, while certain of these materials are found to be conductive to oxygen ions, their conductivity is less than ideal for purposes of gas processing.
Accordingly, it would be a significant advancement in the art to provide materials which are highly conductive to oxygen ions. More particularly, it would be an advancement to provide materials which were highly conductive to oxygen ions and which is relatively easy to handle and easy to incorporate into a gas processing device. It would be a related advancement to provide an apparatus, using such a material, which is capable of separating oxygen from a gaseous mixture. It would also be an advancement in the art to provide highly effective methods for separating oxygen from a gaseous mixture.
Such compositions, methods and apparatus are disclosed and claimed herein.